Filtration Department of Chemical Engineering, By Sidra Jabeen Department of Chemical Engineering, University of Engineering & Technology Lahore

Lecture includes: What is filtration? Understanding of the basic terms How filtration is done? Types of filtration Rate of filtration Filter Media Filter Aid 4/23/2017

Filtration Removal of solid particles from a fluid by passing it through a septum on which the solids are deposited 4/23/2017

1. Solid Particles Concentration Valuability Solubility Size Traces to High % Dissolved Desired Waste Undissolved Ultra filtration Micro filtration Nano Filtration 4/23/2017

2. Fluids A continuous substance which can Flow Liquid Gas 4/23/2017

3. Septum or Filtering Media Membrane Filter Paper 4/23/2017

4. Solid Deposition In the form of cake on the septum Or Trapped inside the pores of the septum 4/23/2017

Example The solid remaining in the filter paper is called the residue. The residue can be dried by spreading it out on the filter paper and allowing the liquid to evaporate. The liquid which has passed through the filter paper is called the filtrate. 4/23/2017

How filtration is done? Fluid flows through the filter medium by virtue of pressure difference across the medium. Through Feed Side Pump Blower Liquid height of column etc Through Product Side By generating Vacuum 4/23/2017

Types of filtration Two basic types of filtration are Cake filtration  Solid particles form a layer of solids on filtering media (called as cake). Deep or Depth bed filtration  Solid particles are caught inside the pores of filtering media. 4/23/2017

1. Cake Filtration Solid particles are retained on septum as a layer of solids called as “cake” 4/23/2017

2. Deep Bed Filtration Further of two types Clarifying filtration Cross flow filtration 4/23/2017

a. Clarifying Filtration Solid particles are trapped inside the septum 4/23/2017

b. Cross flow Filtration Some of the liquid passes through the septum leaving behind conc. solution 4/23/2017

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Factors affecting Rate of Filtration Drop in pressure from the feed to the far side of septum Area of the filtering surface Viscosity of the solution Resistance of the filter cake 4/23/2017

Filter Media Common names Desired Properties Filter Paper Membrane Screen Porous Solids Desired Properties Good Filtration Should not Plug Chemically Resistant Physically Strong Cheap 4/23/2017

Material of construction of filter media Woven Materials (Cotton, Wool, Silk etc) Perforated Metal Sheeting Granular Materials (Sand, Asbestos etc) Porous Solid Felted-Fiber materials (Porous Paper) 4/23/2017

Filter Media Structure Cloth Metal 4/23/2017

Filter Aids Filter aids are granular solids which are used in combination with filtration to enhance rate of filtration. For example, Diatomaceous silica, perlite, purified wood cellulose etc. Filter aids are used in two different ways: Added before filtration As a pre-coat 4/23/2017

Filter Aids Added before filtration This increases the porosity of the cake and reduces resistance of the cake during filtration. As a pre-coat They can be used as a precoat before the slurry is filtered. This will prevent solids from plugging the filter medium and also give a clearer filtrate. Use of filter aids is usually limited to cases where the cake is discarded. 4/23/2017

Types of Filters 4/23/2017

Classification of Filters Filters are classified on following three different basis: Mechanism The driving force Operation

On the basis of mechanism Cake filters Clarifying filters Cross flow filters On the basis of driving force Pressure Filters Vacuum filters Centrifugal filters On the basis of operation Continuous filters Batch or discontinuous filters

Classification of filters Pressure filters  Pressure applied at feed side Vacuum filters  Vacuum applied at product side Centrifugal separators  Spinning the slurry to force separation Continuous filters  Flow of fluid and discharge of solids (cake) is continuous or uninterrupted. Batch filters Flow of fluid is continuous but it has to be interrupted to permit discharging of the accumulated solids. 4/23/2017

Cake filters They can be pressure filters (filter press, shell & leaf filter) Or they can operate under vacuum (rotary drum filter) Or cake filtration can be carried in centrifugal filters (suspended batch centrifuges) Either type can be batch or continuous. 4/23/2017

1.Filter Press Pressure filter operate in batch mode Sets of plates (circular or square) Plate faces covered by septum Slurry enters at one end Solids (cake) collect in chamber and liquid pass out ‘Jammed’ filter Wash liquid, air or steam used to remove residual liquid Press opened and cake removed Washing is time consuming and labor intensive 4/23/2017

Filter Press Pink : Plates Yellow : filter Cloth Blue : Filtrate 4/23/2017

Filter Press 4/23/2017

2. Shell and leaf filter Pressure filter operating in batch mode Stack of leaves inside the closed tank Feed enters in the side of the tank Pass through leaves covered with filter Liquid leaves into discharge manifold Cake is removed by gravity force and vibration Cake is effectively removed and economize labor 4/23/2017

3. Shell and leaf filter 4/23/2017

Shell and leaf filter cake disposal 4/23/2017

3. Rotary Drum Filter Vacuum filter with continuous operation Drum turns at 0.1 – 2 rpm in slurry trough Filter covers the face of the drum Rotary drum filter has four zones: Filtering zone Washing and drying zone Cake removal zone Dead zone Vacuum is applied in first two zones Permeate and washing liquid discharge into separate compartments Doctor blade is used in third zone 4/23/2017

Rotary Drum Filter 4/23/2017

Rotary Drum Filter 4/23/2017

Rotary Drum Filter 4/23/2017

Rotary Drum Filter 4/23/2017

4. Suspended Batch Centrifuge Centrifugal filter operating in batch mode Perforated baskets 750 – 1200 mm in dia. and 18 – 30 inch deep Basket rotates at 750 – 1200 rpm Feed taken from inlet is centrifuged Clear liquid pass out leaving behind solid cake in basket Wash liquid sprayed to remove soluble solids and cake dried For cake removal, basket is turned at 30 – 50 rpm and cake is cut off with unloader knife 4/23/2017

Suspended Batch Centrifuge 4/23/2017

Clarifying Filtration 4/23/2017

Clarifying Filtration Removal of the Small amounts of solids or liquids droplets from the fluids. Principle of Clarification Particles are trapped inside the filter media or on its surfaces Caught by the surface forces Reduce the active dia. of the channel 4/23/2017

Liquid Clarification: Cartridge Filter Series of thin disks with close clearances in a vertical stack Disks carried on a vertical hollow shaft Liquid admitted under pressure Trapping of the solids between disks Liquid flows out through the casing Comb cleaner pass between the disks and drop solids to the bottom of casing 4/23/2017

Cartridge Filter 4/23/2017

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Gas Clarification For atmospheric dust – Pad filters For process dusts – Bag Filters 4/23/2017

Pad Filter Before After Air passed through pads of filter media Pad may be dry or coated with viscous oil For light duty – pads are disposable For heavy duty – reused by rinsing and recoating Before After 4/23/2017

Bag Filter or Bag House Contains one or more large bags of fabric material inside metal casing Dust laden gas enters the filter at the bottom Solid particles trap inside the bag and clean gas leave at the top Solids are discharged by shaking mechanism 4/23/2017

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Cross flow Filtration 4/23/2017

Definition Filtration in which flow of the suspension is parallel to the filtering media used to get concentrated slurry 4/23/2017

Cross flow filtration Operation The feed is passed across the filter membrane at positive pressure relative to the permeate side. A proportion of the material which is smaller than the membrane pore size passes through the membrane as permeate or filtrate; everything else is retained on the feed side of the membrane as retentate. 4/23/2017

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Conventional Filtration Difference b/w cross flow & Conventional filtration Cross Flow Filtration Flow of fluid is parallel to the surface Majority of the feed travels across the surface of the filter Process feed remains in the form of a mobile slurry, suitable for further processing It is possible to fractionate particles by size Conventional Filtration Flow of fluid is perpendicular to the surface Solids in the feed trap inside the filter or retain on the filter Process feed is transformed into solid cake & filtrate It is simply the separation of solids from liquids, no fractionation 4/23/2017

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Performance of cross flow filtration Fouling of the membrane Permeate flux Percent rejection Back flushing Factors for optimizing permeate flux Trans-membrane pressure Shear rate 4/23/2017

Types of cross flow filtration Reverse Osmosis Microfiltration Ultrafiltration Nanofiltration 4/23/2017

Reverse Osmosis The pressure driven transport of solvent from a solution through a membrane is known as reverse osmosis Applications Separating low-molecular-weight substances in solution Concentrating Cleaning wastewater Demineralization 4/23/2017

Points of discussion Osmosis Osmotic pressure Reveres Osmosis 4/23/2017

Micro filtration Used for the particles in the size range of 0.1- 5 μm. Applications Fermentation Biomass clarification and recovery 4/23/2017

Ultra filtration Used for the particles in the size range from 1μm down to 10-3 μm. Applications used in the fractionation of milk as well as protein fractionation 4/23/2017

Nano filtration Used for the particles up to 1nm Nanofiltration is the process with the characteristics b/w those of Ultrafiltration and reverse osmosis. Applications Pharmaceutical processing Water treatment 4/23/2017